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Anyone interested can send me their email and their real name and I'll email back a graph, resulting from actual experiments, depicting the abrupt increase of relative humidity inside a (relatively) air-tight case heated by direct sunlight, and the simultaneous decrease of RH inside an identical case exposed to the same conditions at the same time but left slightly open to allow the heated air to escape. We can debate about how it happened and what to call it, but fact is fact.

When the temperature of air increases, its ability to hold moisture increases. So, when you heat air without introducing additional moisture, the relative humidity must go down. That fact is a fact.

Applying Occam's razor, the simplest and most likely explanation for seeing the increase in RH when you heated the case in your experiment was that there was adsorbed moisture in some material in the case that was released into the atmosphere inside the case when the case was heated.

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When the temperature of air increases, its ability to hold moisture increases. So, when you heat air without introducing additional moisture, the relative humidity must go down. That fact is a fact.

Applying Occam's razor, the simplest and most likely explanation for seeing the increase in RH when you heated the case in your experiment was that there was adsorbed moisture in some material in the case that was released into the atmosphere inside the case when the case was heated.

So then how do you explain the second, identical case, at the same time, place and conditions, that as I mentioned showed a decrease of RH? Keep in mind that I'm talking about multple repeated tests here, with tightly controlled conditions, especially prepared cases and calibrated wireless probes. Each time, the case left slightly ajar saw the RH go down when exposted to direct sunlight, while each time the relatively air-tight case saw RH spike.

While you work on that, please refer to my past post about the Pressure Cooker effect. Occam would approve! But really, GeorgeH, you don't have to believe me. I've already published these observations with favorable peer review.

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So then how do you explain the second, identical case, at the same time, place and conditions, that as I mentioned showed a decrease of RH? Keep in mind that I'm talking about multple repeated tests here. Each time, the case left slightly ajar saw the RH go down when exposted to direct sunlight, while each time the relatively air-tight case saw RH spike.

While you work on that, please refer to my past post about the Pressure Cooker effect. Occam would approve!

Hi Dimitri,

I don’t disbelieve your results - I just don’t think that you can conclude that the increase in relative humidity was due to a so-called “pressure cooker effect” unless you measured the actual pressure increase (if any) inside the cases.

In other words, you need to show that the initial equilibrium RH, temperature, and pressure and the final equilibrium RH, temperature, and pressure are consistent with the respective equilibrium conditions, and that no additional moisture was added to the internal atmosphere during heating.

Furthermore, if the system is truly closed, cooling the case down to the initial temperature and pressure conditions should have returned the RH to the initial value. Did you test that?

Otherwise, without the pressure data, the release of adsorbed moisture from the case materials is the simpler and much more likely explanation for the spike in RH you observed. And the graph you provided supports this.

The green line (closed case) on the graph is consistent with moisture being added to the atmosphere from adsorbed moisture being released by materials inside the case as it is heated. If the pressure and temperature remained constant, then the RH should not have dropped as it did after t~30. If the pressure dropped back to ambient because of slow leaks but the temperature remained high, then the RH should have fallen drastically, but it remains above the initial conditions, further indicating that moisture was added to the atmosphere. If you don't know the temperature and pressure inside the case, you can't tell what is going on.

The redline (open case) should show an immediate drop in relative humidity as it heats up, but it spikes a bit and does not drop below the ambient RH until t~20, which is also consistent with moisture being added to the system from case materials while it heats up.

Unfortunately, the graph does not show the temperature or the pressure at the given time points.

Finally, in order to do this experiment correctly, the cases must truly be “identical” in regards to adsorbed moisture content in the interior materials of the case at the start of the experiment, and that each experiment must be performed under identical climate conditions. Each case must be dry at the start. This would normally be accomplished by heating each case until no more water was detected in the atmosphere, and then cooling in a dry atmosphere.

Did you control for that? If so, how?

I am not saying that your “pressure cooker effect” hypothesis is wrong, only that you have not proven it over a simpler and more likely explanation of your experimental result.

George

P.S. I think your cases are beautiful!

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I don’t disbelieve your results - I just don’t think that you can conclude that the increase in relative humidity was due to a so-called “pressure cooker effect” unless you measured the actual pressure increase (if any) inside the cases.

In other words, you need to show that the initial equilibrium RH, temperature, and pressure and the final equilibrium RH, temperature, and pressure are consistent with the respective equilibrium conditions, and that no additional moisture was added to the internal atmosphere during heating.

Furthermore, if the system is truly closed, cooling the case down to the initial temperature and pressure conditions should have returned the RH to the initial value. Did you test that?

Otherwise, without the pressure data, the release of adsorbed moisture from the case materials is the simpler and much more likely explanation for the spike in RH you observed. And the graph you provided supports this.

The green line (closed case) on the graph is consistent with moisture being added to the atmosphere from adsorbed moisture being released by materials inside the case as it is heated. If the pressure and temperature remained constant, then the RH should not have dropped as it did after t~30. If the pressure dropped back to ambient because of slow leaks but the temperature remained high, then the RH should have fallen drastically, but it remains above the initial conditions, further indicating that moisture was added to the atmosphere. If you don't know the temperature and pressure inside the case, you can't tell what is going on.

The redline (open case) should show an immediate drop in relative humidity as it heats up, but it spikes a bit and does not drop below the ambient RH until t~20, which is also consistent with moisture being added to the system from case materials while it heats up.

Unfortunately, the graph does not show the temperature or the pressure at the given time points.

Finally, in order to do this experiment correctly, the cases must truly be “identical” in regards to adsorbed moisture content in the interior materials of the case at the start of the experiment, and that each experiment must be performed under identical climate conditions. Each case must be dry at the start. This would normally be accomplished by heating each case until no more water was detected in the atmosphere, and then cooling in a dry atmosphere.

Did you control for that? If so, how?

I am not saying that your “pressure cooker effect” hypothesis is wrong, only that you have not proven it over a simpler and more likely explanation of your experimental result.

George

P.S. I think your cases are beautiful!

Thank you George. And I am humble enough to be willing to learn more. After all knowledge always comes with a price! But I can address your issues, as follows:

>I don’t disbelieve your results - I just don’t think that you can conclude that the increase in relative humidity was due to a so-called “pressure cooker effect” unless you measured the actual pressure increase (if any) inside the cases.

...OK, it's theory, I didn't measure the pressure. But one case was left slightly open, and the other was tightly closed. That was the ONLY difference. Both were subjected to heat. Heated air tries to expand, and when it cannot, it goes under pressure. I think that much can be a given.

>In other words, you need to show that the initial equilibrium RH, temperature, and pressure and the final equilibrium RH, temperature, and pressure are consistent with the respective equilibrium conditions, and that no additional moisture was added to the internal atmosphere during heating.

... the last part is easy wnough. I used controlled conditions to make sure that didn't happen. These tests are time-consuming and I don't want to screw up the numbers and invalidated them.

>Furthermore, if the system is truly closed, cooling the case down to the initial temperature and pressure conditions should have returned the RH to the initial value. Did you test that?

... no I didn't, and I wasn't interested in meansuring the pressure.

>Otherwise, without the pressure data, the release of adsorbed moisture from the case materials is the simpler and much more likely explanation for the spike in RH you observed. And the graph you provided supports this.

... no because the cases were identical: Same model, same period of manufacture, same drying-out time. PLUS I have repeated the same test with different cases and obtain the same results.

>The green line (closed case) on the graph is consistent with moisture being added to the atmosphere from adsorbed moisture being released by materials inside the case as it is heated. If the pressure and temperature remained constant, then the RH should not have dropped as it did after t~30. If the pressure dropped back to ambient because of slow leaks but the temperature remained high, then the RH should have fallen drastically, but it remains above the initial conditions, further indicating that moisture was added to the atmosphere. If you don't know the temperature and pressure inside the case, you can't tell what is going on.

... Same model, same period of manufacture, same drying-out time. Same conditions precisely, believe me. What happened is that not enough air leaked out, a minimum of pressure remained and that explains the higher RH values.

>The redline (open case) should show an immediate drop in relative humidity as it heats up, but it spikes a bit and does not drop below the ambient RH until t~20, which is also consistent with moisture being added to the system from case materials while it heats up.

... by moisture being added, do you mean being subtracted from the case itself? If so, remember that both cases were identical so this doesn't invalidate the test.

> Unfortunately, the graph does not show the temperature or the pressure at the given time points.

... see below (at least for the temp).

>Finally, in order to do this experiment correctly, the cases must truly be “identical” in regards to adsorbed moisture content in the interior materials of the case at the start of the experiment, and that each experiment must be performed under identical climate conditions. Each case must be dry at the start. This would normally be accomplished by heating each case until no more water was detected in the atmosphere, and then cooling in a dry atmosphere.

... as a case manufacturer, please grant me that much! My choice was to have a commmon starting point. Mind you, these tests were originally devised for my own research, and not for the purpose of divulgation.

>I am not saying that your “pressure cooker effect” hypothesis is wrong, only that you have not proven it over a simpler and more likely explanation of your experimental result.

... I've encountered the same results consistently, as recently as last month when test comparing similar-weight carbon fiber and sandwich shell cases.

Anyway I don't mind a challenge, it's always an opportunity to learn something.

Cheers!

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George, here's a semi-serious example of what happens when the case is exposed to cycles of heating and cooling!

Semi-serious because not conducted scientifically (although I did sacrifice a $1,000 case) but I was curious to see what would happen when leaving a case on the rooftop in a semi-arid climate for a whole year. Yes, that's mold and fungus. Pretty scary!

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I understand your points, but disagree that the closed case likely remained pressurized (if it ever was really pressurized) throughout the experiment.

By moisture being added inside the case, I am referring to adsorbed water in the case materials vaporizing into the atmosphere inside the case as the case is being heated.

The case did not have to remain pressurized for the additional moisture to remain in the case, keeping the RH high. In fact, if the heated air with the evaporated moisture inside the case was being pushed out of the case as it expanded while it was being heated, and then stopped flowing out once it reached the highest temperature (and the air stopped expanding), it would exactly explain both charts. No pressure build-up is necessary.

In the open case, where there was air flowing in and out of the case, the additional moisture would leave pretty quickly through convection as cooler air flowed in and hotter air flowed out and equilibrium pressure. Once the air with the additional moisture was gone, the RH in the hot case would drop and stay down (as it did).

So pressure does not have to be invoked at all to explain the results of either of your charts, or to understand why the results are repeatable.

Frankly, I don't think that you considered the release of adsorbed water from the case materials when you heated them as contributing to the RH, but it certainly would. Seemingly dry materials, such as glass, can adsorb a lot of water! Many other materials can, too. Most people don't consider how wet the world really is, but when you try to run processes using highly moisture-sensitive catalysts, you find out pretty quickly that there is water in and on many surfaces.

If a violin case was truly a pressure vessel, then closing a case in a warm environment and moving to a cold environment would cause the case to become negatively pressurized. This could cause the walls to pull in (not good!) and/or make the case difficult to open. I have never heard of or seen this happen.

Anyway, I think we can all agree that keeping cases cool is important, and that it is preferable to have a case that breathes well than a case that doesn't.

1 hour ago, Dimitri Musafia said:

George, here's a semi-serious example of what happens when the case is exposed to cycles of heating and cooling!

Yikes!

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Thank you for taking the time to post here. I'm glad to see that you test your cases.

I concur with GeorgeH on all points. The extra moisture in the air when you heated the case is attributable to water adsorbed on the materials in the case. If you exclude experimental error, then since there is no other source of water, there is no other possibility.

16 hours ago, Dimitri Musafia said:

So then how do you explain the second, identical case, at the same time, place and conditions, that as I mentioned showed a decrease of RH? ...Each time, the case left slightly ajar saw the RH go down when exposted to direct sunlight, while each time the relatively air-tight case saw RH spike.

It should not be a surprise that when you opened the case, moisture escaped and the air was exchanged with outside air, thereby dropping the relative humidity. Because the case was heated, the relative humidity dropped below the ambient, initial value.

I find it odd that you describe your own cases as behaving like a pressure cooker unless one purchases a special option. I wouldn't characterize it that way, especially when one remembers that a pressure cooker contains high-pressure, high-temperature steam that might kill you if you succeed in removing the top when it's hot. If you prefer to describe your own cases in those terms, it's fine with me, however incorrect it may be.

I also think it's fine to state why you think your cases are superior. Every manufacturer does that. I do, however, object when people on this forum take advertising copy and state it as fact, using it to denigrate competitors without evidence. That's probably your fault, but I do object. I really don't know the relative merits of the various brands of cases, so I neither endorse nor disendorse any brand.

By the way, the answer to the problem above, first posed by WorksAsIntended? (The one that no one answered.) I'll give the answer in round numbers, and this time I'll use English units for our American friends. Assuming a constant volume, closed system (completely sealed, containing air only) with a temperature rise from 20°C to 80°C gives an increase in pressure from about 15 pounds/square inch (at atmospheric pressure) to 18 pounds/square inch. Thus, the difference between the inside and the outside of the case is 3 pounds/square inch. The total force is the pressure difference times the area of the opening, which for a small case is about 200 square inches. Thus, the total force is 3 x 200 = 600 pounds. For a rectangular case it would be about 50% higher, thus 900 pounds (409 kg)! I think you can safely assume that that would strain the latch. In fact, it might blow the lid off when you try to open it. But I think an increase of 60°C is too high. Let's use your figure of 20°C. That means the force is only 300 pounds (91 kg). That kind of stretches credulity, don't you think? Since the calculation is correct, that means one or more assumptions is incorrect. If you really want to know, these cases do not hold pressure. I work with vacuum and pressure systems in the lab, and it's really rather hard to seal something, especially with rough materials used in a violin case. I would be astonished if any violin case holds pressure or a vacuum, although in the case of plastic materials, I could be persuaded with evidence. I do emphasize, however, that most cases keep atmospheric moisture in or out, to a certain extent.

As for your case with the mildew, I do agree that thermal cycling will bring in a certain amount of moisture, perhaps sufficient to cause a lot of damage. Every time the case cools, the air inside will contract, and it will bring in moisture from the outside that can be used by mould. A pressure port will do nothing to solve that problem, and I would be astonished if it were tested and found to reduce mildew damage. I could be wrong about this, but the kind of damage that you showed suggests to me that rainwater might have penetrated the case.

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So pressure does not have to be invoked at all to explain the results of either of your charts, or to understand why the results are repeatable.

Frankly, I don't think that you considered the release of adsorbed water from the case materials when you heated them as contributing to the RH, but it certainly would. Seemingly dry materials, such as glass, can adsorb a lot of water! Many other materials can, too. Most people don't consider how wet the world really is, but when you try to run processes using highly moisture-sensitive catalysts, you find out pretty quickly that there is water in and on many surfaces.

Like I said, the cases were carefully chosen to be identical in terms including water content. They were a model that has limited moisture-absorbing velvet compared to other models and had been built, together, several months previously, meaning they were dried out. And, lastly, the tests have been repeated, obtaining the same results, even with cases by other manufacturers, made of different materials.

Now, since I believe the pressure build-up to be the key issue, I experiemnted with two more identical cases, one being stock and the other having pressure-release ports built in. These ports are basically holes which can allow excess pressure to escape, but not water to get inside when it rains.

You'll see that during the test the RH spike was basically reduced by half in the case that had the pressure release ports. Had they been larger, ot there had been more of them, it is likely that the relative humidity could have been maintained at a stable level throughout the experiemnt.

Check out the graph:

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I also think it's fine to state why you think your cases are superior. Every manufacturer does that. I do, however, object when people on this forum take advertising copy and state it as fact, using it to denigrate competitors without evidence. That's probably your fault, but I do object. I really don't know the relative merits of the various brands of cases, so I neither endorse nor disendorse any brand.

If I have denigrated a competitor on this forum, or "stated that [Musafia] cases are superior" to others, please show me where and I will delete it immediately. I take special care not to mention my brand nor my products as I agree 100% it is poor etiquette at best. I even took the time to remove my brand from the graphs I posted.

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You'll see that during the test the RH spike was basically reduced by half in the case that had the pressure release ports.

That's plausible, and somewhat more effective than I would have expected. However, it has nothing to do with total pressure, and everything to do with allowing moisture to escape. Technically, the vent allows a reduction in partial pressure of water on the order of 0.5 kPa (0.8 psi), but probably no change in total pressure.

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If I have denigrated a competitor on this forum, or "stated that [Musafia] cases are superior" to others, please show me where and I will delete it immediately. I take special care not to mention my brand nor my products as I agree 100% it is poor etiquette at best. I even took the time to remove my brand from the graphs I posted,

There were such allegations in this thread, but not by you. I have no quarrel with the way you have conducted yourself. The claims of superiority are on your web site, and perfectly proper. And, as GeorgeH stated, nice cases. I even bought one myself.

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There were such allegations in this thread, but not by you. I have no quarrel with the way you have conducted yourself. And, as GeorgeH stated, nice cases.

Thank you - appreciated.

I'm just trying to engage other people interested in the same problems for some interesting conversation and hopefully learn something. I appreciate the input from all, especially from those of you who disagree because that means I have to work harder to back up my theories, and in the process, perhaps discover something new.

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Yes, I'm actually rather suprised that cases absorb and release so much moisture.

My latest curiosity is to see what influence an effective case humidifier can have under these circumstances. The humidifiers I've used contain 10g of water and release it through evaporation via 210mm2 holes.

I've done some testing but unfortunately my data is incomplete at this time, and we no longer have the climatic conditions here to continue with any uniformity of numbers. Maybe this is more appropriate for a separate thread. I'd be interested to know if anyone else has done such experiments.